Within the past few decades the use of membranes in fluid separations has developed to a considerable degree. In this technology permeable membranes capable of selectively separating one or more components from a feed mixture of at least two materials, either liquid or gas, have been used to construct the permeators or modules which house the membranes. Initially the permeators were of relatively simple construction and used a single selective membrane to recover two streams, a permeate stream and a raffinate stream, from the feed. In most instances the permeable membrane chosen was one which would provide an enriched permeate stream of the desired component while the raffinate stream had a diminished content of the permeated component. However, one could have a system in which the reverse was true. These systems have a single permeable membrane capable of separating a component from other components in the feed. The single membrane permeators permitted recovery of only two separate streams. Such permeators are exemplified in U.S. Pat. Nos. 3,133,132 issued May 12, 1964 to S. Loeb et al., 3,442,002 issue May 6, 1969 to J. E. Geary, Jr. et al., 3,794,468 issued Feb., 26, 1974 to R. J. Leonard, 4,207,192 issued June 10, 1980 to M. J. Coplan et al., No. 4,430,219 issued Feb., 7, 1984 to H. Kuzumoto et al., to mention but a few instances.
In the 1970's a new concept was presented, the concept of using two, generally different, permeable membranes in a permeator system for use in processes in which more than two components or product streams were to be recovered. This concept has led to several modifications. In one concept the permeators were connected in series with a different permeable membrane in each permeator, as exemplified in U.S. Pat. No. 4,140,499 issued to Ozaki et al. on Feb., 20, 1979, which also provided for the recycle of a portion of the component from at least one of the streams from a subsequent permeator to a previous permeator in the series. In another modification a cascade system was described wherein a greater multiplicity of permeators and recycles was employed, as exemplified in U.S. Pat. No. 4,119,417 issued to Heki et al. on Oct. 10, 1978.In both the Ozaki et al. and Heki et al. systems described, each permeator cell contained but a single permeable membrane in the individual permeator cell or cartridge.
These modifications led to a still further modification wherein each permeator cell contained two different types of permeable membranes, each membrane of different permeability characteristics being selectively permeable by a different fluid component originally present in the feed mixture and each permeable membrane being separately constructed as a distinct unit. These permeator cells or systems are now known as multimembrane permeators; they are permeator cells constructed of two different permeable membranes each capable of separating different components from a feed mixture, the two membranes being present in the single permeator cell. In the multimembrane permeators the feed is simultaneously contacted with different membranes and one recovers two separate permeate streams, a first stream enriched in a first component and a second stream enriched in a second component. Illustratively, a single multimembrane permeator cell in a permeator module can be used to separate or enrich the components from a multicomponent feed mixture; it must be accepted that the permeable membranes employed have the selective properties needed to achieve the desired separations. In the typical operation there is enrichment, not complete separation of specific gases.
One of the major problems with multimembrane permeator modules has been the commercial construction of satisfactory unified multimembrane permeation cells and until this date they have been difficult to produce. Though several procedures have been published none have been commercially practical.
In an article by Ohno et al., J. Nucl. Sci. & Tech., 14, 589 (1977), two membrane permeators are disclosed in which the separation cell contains two different kinds of permeable membranes, as shown in FIG. 7 of the article. Each permeable membrane cell unit, however, is separately constructed and contains only a single permeable membrane. Thus there is one membrane unit that has a porous membrane and another membrane unit that has a nonporous membrane, the two membranes differing in gas permeability. The two different permeable membrane units are then used to construct the described two membrane permeator and a multiplicity of the two-membrane permeators is used in a cascade system to separate gas mixtures. The two membrane permeators or cascades of the two-membrane permeators described by Ohno et al. were usually used to provide a superior separation factor for a single component that was present in the feed mixture. Ohno et al. did not consider recovery of two separate enriched permeates, each of a different component, they were concerned with the separation of krypton from a nitrogen-krypton mixture. Further, nowhere in the article do the authors suggest or disclose co winding two or more permeable membranes to form a unified multimembrane permeation cell as hereinafter defined and the use of this unified multimembrane permeation cell to make a unified multimembrane permeator module as hereinafter defined. Separate membrane units each having a different permeable membrane were prepared and then sets of these were used to make their described two-membrane permeator.
U.S. Pat. No. 4,119,417 and U.S. Pat. No. 4,140,499, supra, mention separation cells having two kinds of membranes (col. 1, lines 18-19). In both instances, they use the term separation cell to refer to the two celled permeator unit of FIG. 2 in which each cell contains a single different permeable membrane. In the methods disclosed, a plurality of two celled separating units is used in a multi-stage series, and a two-celled unit comprising a first separating cell provided with a membrane and a second separating cell generally provided with a different membrane is employed. The series arrangements are used for gas separating. The patents make no reference to unified multimembrane permeation cells in which two different permeable membranes are co-wound to form the unified multimembrane permeation cell, or the unified multimembrane permeator module of this invention.
Ohno et al., "Separation of Rare Gases By Membranes", Radiochem. Radioanal. Letters, 27, 299 (1976), was an early disclosure of a new separation cell which had two compartments, each compartment having different separation functions attained by the use of different membranes in each compartment. There is no suggestion of the unified multimembrane permeation cell of this invention with the two different hollow fiber permeable membranes wound together in a single unified multimembrane permeation cell.
Sirkar in "Asymmetric Permeators - A Conceptual Study", Sep. Sci. & Tech. 15, 1091 (1980), studied the permeator concept of Ohno et al. in multicomponent gas separations and the various applications in which the Ohno et al. permeators could be used. Among the systems referred to are those in which different membranes are connected to the opposite tube sheets in the configuration shown by his FIG. 2. Other possibilities discussed are hollow fibers of different membranes evenly dispersed amongst each other instead of each type being bundled separately. He states one could make the lengths of the hollow fibers of one material such that they get sealed in the tubesheet meant for the other membrane material. However, he gives no indication how these structures can be produced. Another possible arrangement disclosed by Sirkar is a stack of parallel membranes of differing permeability with suitable spacers in between to provide chambers as shown in FIG. 3.
Stern et al. "Recycle and Multimembrane Permeators for Gas Separations" J. Memb. Sci., 20, 25 (1984), reviewed the use of different membranes in different permeator configurations for gas separations. In their study each permeator cell contained a different type of permeable membrane; they did not use permeators in which two or more different membranes were wound to produce a unified multimembrane permeation cell as hereinafter defined in this invention. Permeators discussed by Stern et al. were then considered in various arrangements, in series, in parallel, and in the same vessel, as shown in their FIG. 10, and their conclusion was that best results would be achieved when the two different permeable membranes were contained in the same vessel or permeator module. However, in no instance did they disclose or suggest a permeator containing two or more different membranes wound together to form the unified multimembrane permeation cell of this invention.
The use of multiple membrane permeators to separate multicomponent gas mixtures into three product streams using two different membranes preferentially selective to two different components of the mixture was studied by Sengupta et al., "Multicomponent Gas Separation By An Asymmetric Permeator Containing Two Separate Membranes", J. Memb. Sci., 21, 73 (1984). This study involves different flow patterns and mathematical equations were formulated; but, nowhere did it discuss the method used for constructing the multiple membrane permeators.
Perrin et al., "Modeling of Permeators with Two Different Types of Polymer Membranes", AIChE J., 31,1167 (1985), discuss flow patterns and develop mathematical models for gas separations in which two types of membranes are enclosed in the same permeator module. The systems they employed did not intersperse two membranes, each membrane was maintained apart from the other even though the two membrane units may be enclosed in the same vessel or permeator module. Perrin et al. nowhere disclose how to construct the units.
Sengupta et al. "Ternary Gas Mixture Separation In Two-Membrane Permeators" AIChE J., 33,529 (1987), studied the single stage separation of multicomponent gas mixtures in a hollow fiber permeator module which simultaneously housed two different types of permeable membranes, cellulose acetate and silicone rubber, to separate the feed into three streams, two permeates and one reject, each stream enriched in a different component. In the permeator disclosed the two permeable membranes were potted, or housed, together inside a shell or module with the ends separated from each other so that the permeates could be collected individually, as shown in FIG. 2. The reference does not suggest or disclose a unit in which two different permeable membranes are wound together to form the unified multimembrane permeation cell of this invention.
In "Separation of a Helium Methane Mixture in Permeators with Two Types of Polymer Membranes", Perrin et al., AIChE J., 32,1889 (1986), disclosed and used two-membrane permeators. In the first column and in Table 1 on page 1891, six different permeator modules evaluated were discussed, three permeator modules were constructed solely of silicone rubber and three solely of cellulose triacetate. In the second column and in Table 2 on page 1891 permeator modules that enclosed two different types of permeable membranes intermixed with each other are discussed and their structure is shown in FIG. 2. The authors state, and FIG. 2 shows, that in the construction of the two membrane permeator one of the tubesheets, or headers, was provided with dual outlets for two permeate product streams instead of a single stream. The authors say construction was similar to a single-membrane permeator but in the reference specifically lacking is any suggestion or disclosure of a unified multimembrane permeation cell of this invention with outlet for one membrane in one tubesheet and for the other membrane in a second tubesheet or of the method for its construction.
U.S. Pat. No. 4,207,192, issued on June 10, 1980 to M. J. Coplan et al., discloses methods for fabricating hollow filament separatory cells and modules from a single type of permeable membrane. The patent teaches that modules may be constructed wherein either one end or both ends of the bore can be opened. However, at no time is there any suggestion or disclosure of fabricating a unified multimembrane permeation cell or of a unified multimembrane permeator module having two or more different permeable membranes in the cell as disclosed in this invention, nor of any method for achieving this.
None of the references contain any suggestion or disclosure of a unified multimembrane permeation cell or of a method for producing a unified multimembrane permeation cell or unified multimembrane permeator module as hereinafter defined containing two or more permeable membranes in the unified multimembrane permeation cell.